It is well known that telecommunication signals can be transmitted over long hauls in digitally encoded form. Approximately in 1962, the T1 carrier system began to serve the U.S. metropolitan areas in conjunction with the D1 Digital Terminal System (DTS), and since then improved and more versatile digital terminal systems have been introduced.
The basic elements of a digital terminal system are the modulation, transmission, reception, demodulation, signaling, and testing subsystems. To perform the modulation and demodulation functions efficiently, the D1 system and all subsequent D type system (D2 through D4) have been organized in channel banks of two 12 channel groups, collectively called a digroup, with common control extended over the entire digroup. Modulation is performed by developing pulse amplitude samples of voice frequency signals and encoding the PAM samples into 8 bit PCM words. The PCM words of individual voice frequency circuits, or channels are time division multiplexed and the resultant multiplexed signal is applied to the digital transmission medium.
Although intially the D type channel banks are intended as interface elements between analog voice circuits and digital transmission facilities interconnecting central offices, their primary use changed substantially over the years. More and more the switching machines were converted to digital operation with stored program control and those machines employ direct interface equipment to link the T-carrier systems. These developments shifted the primary application of channel banks from handling interoffice message trunks to providing special service circuits.
One example of a special service circuit is one that allows a call to be made outside the local exchange area for the price of a local call. Another example is providing a digital computer-to-computer link.
One characteristic of special circuits (which is closely related to the nature of their use) is that they often are being rearranged. Unlike the message trunks, about twenty five percent of all special-service circuits are either installed, modified, or disconnected every year. This churning is expected to reach fifty percent in the near future.
Each of the circuits for these special services lines must be designed, set up electrically and tested before being turned over to a customer. This requires a substantial dedication of human resources with the existing D4 channel bank because there are many types of circuits, many different electronic modules that collectively create the circuits, and the electricl set-ups and testing of such circuits require substantial coordination between operating company personnel.
To overcome the labor intensive provisioning and testing operations, to enhance operation of the channel bank and to reduce costs, the D5 Channel Bank system was designed. From a macroscopic view and with reference to FIG. 1, the D5 channel bank includes a system controller 10 and a plurality of channel banks 20. The system controller provides access for local craft personnel and communication links to other systems and local craft persons, as well as control signals and test signals, via links 11, to channel banks 20.
Channel banks 20 comprise a controller, metallic access means, a clock unit, a power unit, a facility interface unit, and four digroups. Each digroup contains up to 24 channel units.
It is an object of this invention to provide channel units that are capable of interacting with controller 10 in a robust, error free, manner.